We report on theoretical and experimental investigations into electrostatic chuck designs for use in future e-beam
lithography on 450 mm Silicon wafers. Ultra-low thermal expansion glass (ULE) and Si infiltrated Silicon Carbide
(SiSiC) designs were evaluated by finite element modeling, subject to a mass budget of 8 kg. In addition to massive
chucks, light-weight designs were created by applying bore holes through the chuck body below its surface.
Considerable chuck bending under gravity is observed with classical kinematic 3-point mounts. Out-of-plane distortions
of about 1250 (650) nm and 400 (200) nm for the massive and light-weight designs of ULE (SiSiC), respectively, were
calculated. The corresponding surface in-plane distortions for a chucked Si wafer of standard thickness 925 μm amount
to about 3 (1.6) nm for the massive and 1 (0.5) nm for light-weight designs of ULE (SiSiC), respectively. By using the
standard 6<sup>th</sup> order polynomial correction upon e-beam writing, these values can be reduced to ≤0.7 nm for the massive
designs with both materials. Various pin-pattern configurations for an ideally flat chuck surface were adopted to
determine resulting wafer bending under the influence of electrostatic forces. At a typical electrostatic pressure of about
18 kPa, a square pin pattern of pin-pitch 3.5 mm and pin-diameter 0.5 mm results in wafer in-plane distortions <0.5 nm,
which is considered tolerable for obtaining the desired total overlay accuracy of <4 nm. The pin structure manufacturing
process for a corresponding ULE chuck surface was experimentally tested and verified. A nearly elliptic ULE plate,
slightly larger than the wafer, was structured with a Chromium hard-mask and subjected to low pressure reactive ion
etching to generate the pin-pattern. A homogeneity of about 7 % was obtained for the etching process, which is fully
sufficient with respect to resulting variations in electrostatic attraction.
The reflection of back-scattered electrons (BSE) at the objective lens of an electron beam writer leads to a diffuse resist
exposure which extends over several millimetres. The deposed energy of this unintentional exposure is much lower than
the direct one. However, if the area of the direct electron beam exposure is large enough the accumulated energy is no
longer negligible and may cause significant CD variations. Therefore, it is of crucial importance to study possible ways
of reducing this dose contribution to a minimum and in order to perform a correct proximity correction targeting to
determine its radial distribution.
In this work a model of a 50kV E-Beam writer was developed, consisting of a resist-coated silicon wafer and an opposing low-reflection disk mounted at the pole piece of the objective lens. In order to improve the low-reflection disk, different material compositions as well as an optimized surface topography of the disk are modelled.
In this work, we investigated possible geometry optimizations of backscattered electron (BSE) detectors in order to significantly improve the signal to noise ratio (SNR) of shallow Si-topographic marks. To achieve this, Monte Carlo simulations of the BSE angular distribution as well as of the BSE exit position were performed. A comparison of some theoretical calculations with the respective experimental results allowed us to qualify the theoretical results. Based on these results, we are able to present an optimized BSE detector design featuring a significant improvement of the measured SNR.
Proc. SPIE. 6792, 24th European Mask and Lithography Conference
KEYWORDS: Signal to noise ratio, Lithography, Electron beams, Optical lithography, Optical alignment, Electron beam direct write lithography, Semiconducting wafers, Signal detection, Wafer testing, Overlay metrology
With shrinking dimensions in the semiconductor industry the lithographic demands are exceeding the parameters of the
standard optical lithography. Electron beam direct write (EBDW) presents a good solution to overcome these limits and
to successfully use this technology in R&D as well as in prototyping and some niche applications. For the industrial
application of EBDW an alignment strategy adapted to the industrial standards is required to be compatible with optical
lithography. In this context the crucial factor is the overlay performance, i.e. the maturity of the alignment strategy under
different process conditions. New alignment marks improve the alignment repeatability and increase the window of the
signal-to-noise ratio towards smaller or noisier signals. Particularly the latter has proved to be a major contribution to a
higher maturity of the alignment. A comparison between the double cross and the new Barker mark type is presented in
this paper. Furthermore, the mark reading repeatability and the final overlay results achieved are discussed.
Proc. SPIE. 6921, Emerging Lithographic Technologies XII
KEYWORDS: Signal to noise ratio, Lithography, Sensors, Silicon, Monte Carlo methods, Optical alignment, Electron beam direct write lithography, Semiconducting wafers, Signal detection, Vestigial sideband modulation
New types of alignment marks to be applied in electron beam direct write (EBDW) have been studied theoretically and
The dependence of signal contrast and signal form on such mark properties like step height, mark pitch and stack
material has been investigated in detail using Monte Carlo simulations.
The different alignment marks were etched in Si to different depths and the respective alignment repeatability was determined
with a Vistec SB3050 DW lithography tool. Finally, for the most promising mark, test exposures were performed
and the overlay determined.